It being on the GCD isn't inherently a problem (plenty of time to stun puddings in A3S, plenty of time to stun legs in A4S) - it's because of the interrupting combos and high TP cost bits that makes it being on the GCD a crux. It's actually amazing for the hand in A3S, as an example, but the TP cost is too severe for it to work properly. Keeping it on the GCD, lowering the TP cost, and making it not interrupt combos would be the best solution I think.